Performance Evaluation of Stormwater Drainage System: The Case of Hawassa City
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Date
2019-03
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Addis Ababa University
Abstract
The main objective of this study was to investigate the existing Hawassa city stormwater drainage
problems with respect to its hydraulic performance. The methodological approaches employed in
this study include; the identification of the existing stormwater drainage network flow direction,
and delineation of contributing areas. Field surveying work was conducted to inspect the existing
drainage condition and to measure channel geometric dimensions. In order to develop the Intensity
Duration Frequency curve (IDF) the rainfall intensities for different duration were analyzed for a
period of 1985-2016. Probability distribution functions were analyzed and identified using
Microsoft Excel.
The analysis from the frequency distribution indicate that General Extreme Value Type I (GEV-I)
distribution was fitted well reasonably for the rainfall intensities. Rainfall quantiles for a duration
of 24hr with a magnitude of 52.75, 68.67, 79.20, 92.51, 102.4 and 112.2 mm/hr was predicted for
a return period of 2,5,10,25, 50 and 100 years respectively. Based on the flow direction of the
study area the existing drainage channel capacity were checked by using flow master software.
Established on settlement of the city about 66, sub-catchments were delineated in the existing
system and by using the intensity estimation (T=10 years) the hydrological peak flow determined
by using Bentley CivilStorm software. The rational formula is used to compute the peak flood
estimation for (T= 10).
The result from model compared with existing hydraulic capacity of the channels and it shows that
at Wanza square stormwater drainage were not sufficient to carry the expected runoff during
rainfall events. In general, the flood problems identified at Wanza square and Wanza square to Old
bus station were mainly due to in adequacy of the drainage channel capacity and further
substantiated by the waste disposal from different sources into the drainage channels. To resolve
these problems BMP’s have been recommended. Finally, the analysis from this study permits
future work on stormwater drainage design should strictly follow the standards of hydrologic and
hydraulic design and social and environmental considerations.
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Keywords
stormwater, stormwater drainage, runoff, channel capacity